The present invention relates to a sheet feeder provided in an image forming device such as a copy machine, laser printer, and facsimile machine, and more particularly, to a sheet feeder for feeding stacked sheets of a recording medium one sheet at a time.
A conventional sheet feeding device provided in image forming devices such as copy machines, laser printers, and facsimile devices is shown in FIG. 1. This sheet feeding device is provided with a feed roller 1 for conveying a paper 2 and a separating pad 3 opposing the feed roller 1 such that the paper 2 is interposed between the feed roller 1 and separating pad 3. This sheet feeding device is well known in the art for separating and conveying each of a stack of paper 2 one sheet at a time. The separating pad 3 shown in FIG. 1 is embedded into a depression formed in a sheet receiving portion 5 of a separating pad holder 4. The separating pad holder 4 is urged toward the feed roller 1 by a spring 7 interposed between a fixed stop member 6 and the bottom surface of the sheet receiving portion 5. The feed roller 1 is formed of an elastic material in an approximate D-shape and includes a circumferential surface 8 for contacting and conveying the paper 2 and a cut-out surface 9 formed by cutting away a portion of the circumferential surface 8. Rollers 10 are rotatably mounted on both ends of the feed roller 1. Both of the rollers 10 are formed with a radius slightly smaller than the radius of the circumferential surface 8. Hence, when the circumferential surface 8 is conveying the paper 2, the rollers 10 do not contact the paper 2. However, when the circumferential surface 8 has completed conveying the paper 2 and the cut-out surface 9 opposes the paper 2, the peripheral surface of the two rollers 10 contact the paper 2 and oppose the urging force of the separating pad 3, thereby forming a space between the cut-out surface 9 and the paper 2 to prevent the cut-out surface 9 from contacting the paper 2.
Guide rollers 12 and 13 are positioned downstream of the feed roller 1 and convey the paper 2 after the circumferential surface 8 has completed conveying the paper 2. The cut-out surface 9 is separated a prescribed distance from the paper 2 by the two rollers 10, thereby stopping driving of the paper 2. In addition, the rollers 10 contact and rotate on the top surface of the paper 2, which is conveyed by the guide rollers 12 and 13, thereby allowing only one paper 2 to be conveyed at a time.
However, when conveying the paper 2, the circumferential surface 8 and the rollers 10 contact the paper 2 and separating pad 3 at different positions, or heights, because the radius of the two rollers 10 is smaller than the radius of the circumferential surface 8. This difference in radii creates a bump when the paper 2 and separating pad 3 change from contacting the rollers 10 to contacting the circumferential surface 8 and vice versa, causing the separating pad 3 to move upward and downward.
When the separating pad 3 moves upward and downward, as described above, there is a tendency for a plurality of paper 2 to be fed simultaneously, causing doubles or partly overlapped sheets to be fed. In other words, when the paper 2 is first fed under the feed roller 1, the leading edge of the paper 2 contacts an entry contact portion 15 of the circumferential surface 8, which is the transition point from the rollers 10 contacting the separating pad 3 to the circumferential surface 8 contacting the separating pad 3. Therefore, the separating pad 3 is moved downward. In this instant, the pressure on the paper 2 changes greatly, inviting a plurality of paper 2 to enter simultaneously between the circumferential surface 8 and separating pad 3, inviting the feeding of doubles or multiple sheets. Also, when the circumferential surface 8 has completed conveying the paper 2, an exit contact portion 14 of the circumferential surface 8 is the last part of the feed roller 1 to contact the paper 2 before the roller 10 come in contact with the paper 2. In the instant that the paper 2 separates from the circumferential surface 8, the separating pad 3 moves upward, causing a great change in pressure on the paper 2, inviting the next paper 2 or plurality of next paper 2 to be conveyed along with the paper 2 being conveyed between the guide rollers 12 and 13, inviting the feeding of partly overlapped sheets. That is, immediately before stopping rotation of the sheet feed roller 1, the front part of the sheet reaches the guide rollers 12, 13, and the sheet is fed by the guide rollers 12,13. Further, the pressure from the separation pad 3 to the sheet is temporarily lowered when the paper 2 and separating pad 3 change from contacting the circumferential surface 8 of the feed roller 1 to contacting the rollers 10. In this case, the subsequent sheet may also be fed in accordance with the advancing movement of the precedent sheet due to the frictional force between the precedent sheet and a subsequent sheet.